open of /usr/lib/pgsql/data/pg_clog/0F3E failed

Hi:

I'm running a large database on PostgreSQL 7.1.3. 20 days ago the
database failed with a threatening and not too descriptive error like:

pg_exec() query failed:  server closed the connection unexpectedlyThis
probably means the server terminated abnormally                 before or while
processing the request.

I lost some data and had to recreate a table, without knowing the reason
for the error, now I got this error:

FATAL 2:  open of /usr/lib/pgsql/data/pg_clog/0F3E failed: No existe el
fichero o el directorio
server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.

When I VACUUM the affected table I get:

FATAL 2:  open of /usr/lib/pgsql/data/pg_clog/0F5C failed: No existe el
fichero o el directorio
server closed the connection unexpectedly
         This probably means the server terminated abnormally
         before or while processing the request.
The connection to the server was lost. Attempting reset: NOTICE:
Message from PostgreSQL backend:
The Postmaster has informed me that some other backend
died abnormally and possibly corrupted shared memory.
I have rolled back the current transaction and am
going to terminate your database system connection and exit.
Please reconnect to the database system and repeat your query.
Failed.

I guess I have lost again some data in one table.

Someone can help with the origin of these errors? How can I avoid them?

Thanks in advance.